Space exploration is our human response to curiosity about Earth, the moon, the planets, the sun and other stars, and the galaxies. Piloted and unpiloted space vehicles venture far beyond the boundaries of Earth to collect valuable information about the universe. Human beings have visited the moon and have lived in space stations for long periods. Space exploration helps us see Earth in its true relation with the rest of the universe. Such space exploration could reveal how the sun, the planets, and the stars were formed and whether life exists beyond our own world.

The space age began on Oct. 4, 1957. On that day, the Soviet Union launched Sputnik (later referred to as Sputnik 1), the first artificial satelliteto orbit Earth. The first piloted space flight was made on April 12, 1961, when Yuri A. Gagarin, a Soviet cosmonaut, orbited Earth in the spaceship Vostok (later called Vostok 1).

Unpiloted vehicles called space probes have vastly expanded our knowledge of outer space, the planets, and the stars. In 1959, one Soviet probe passed close to the moon and another hit the moon. A United States probe flew past Venus in 1962. In 1974 and 1976, the United States launched two German probes that passed inside the orbit of Mercury, close to the sun. Two other U.S. probes landed on Mars in 1976. In addition to studying every planet except Pluto, space probes have investigated comets and asteroids.

The first piloted voyage to the moon began on Dec. 21, 1968, when the United States launched the Apollo 8 spacecraft. It orbited the moon 10 times and returned safely to Earth. On July 20, 1969, U.S. astronauts Neil A. Armstrong and Buzz Aldrin landed their Apollo 11 lunar module on the moon. Armstrong became the first person to set foot on the moon. United States astronauts made five more landings on the moon before the Apollo lunar program ended in 1972.

During the 1970's, astronauts and cosmonauts developed skills for living in space aboard the Skylab and Salyut space stations. In 1987 and 1988, two Soviet cosmonauts spent 366 consecutive days in orbit.

On April 12, 1981, the United Statesspace shuttle Columbia blasted off. The shuttle was the first reusable spaceship and the first spacecraft able to land at an ordinary airfield. On Jan. 28, 1986, a tragic accident occurred. The U.S. space shuttle Challenger tore apart in midair, killing all seven astronauts aboard. The shuttle was redesigned, and flights resumed in 1988. A second tragedy struck the shuttle fleet on Feb. 1, 2003. The Columbia broke apart as it reentered Earth's atmosphere, killing all seven of its crew members.

In the early years of the space age, success in space became a measure of a country's leadership in science, engineering, and national defense. The United States and the Soviet Union were engaged in an intense rivalry called the Cold War. As a result, the two nations competed with each other in developing space programs. In the 1960's and 1970's, this "space race" drove both nations to tremendous exploratory efforts. The space race had faded by the end of the 1970's, when the two countries began to pursue independent goals in space.

A major dispute in the development ofspace programs has been the proper balance of piloted and unpiloted exploration. Some experts favor unpiloted probes because they may be cheaper, safer, and faster than piloted vehicles. They note that probes can make trips that would be too risky for human beings to attempt. On the other hand, probes generally cannot react to unexpected occurrences. Today, most space planners favor a combined, balanced strategy of unpiloted probes and piloted expeditions. Probes can visit uncharted regions of space or patrol familiar regions where the data to be gathered fall within expected limits. But in some cases, people must follow the probes and use human ingenuity, flexibility, and courage to explore the mysteries of the universe.